This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. FPOP originally uses fast photolysis to produce radicals for footprinting proteins and their complexes. The original reagent we developed for FPOP was OH radical produced by photolysis of hydrogen peroxide with an excimer laser. In this project we are exploring other sources to produce radicals with different reactivity and selectivity, and even different modifications. Radical under investigation include carbonate radical anion, iodine radical. We are mapping out the reactivities and selectivities of these radicals and looking for other radical species to use in FPOP.